That question's been nagging at me for years now. There's a large subjective, one might say philosophical component, to the judgment that the colors look "right" in a given image.

A few years ago I took some Viking Lander images showing frost and produced color images calibrated on the assumption that the frost was white. Several people since then have pointed out that what I probably accomplished was to subtract the genuine red contributed by dust in the atmosphere; and this is what gets somewhat philosophical: Human eyes might see everything tinged with red, but a hypothetical vision system evolved on Mars would naturally compensate, normalize the spectrum so that subjective "white" produced the same impression to Martian eyes on Mars as "white" produces in human eyes on Earth. In a sense, it's a technique for producing images as seen through hypothetical Martian eyes.

The rocks in my Viking images were a very similar salmon/beige to the rocks in your first image. More interesting is the green in yours, which seems to closely match the shade of green shown in a majority of "true color" orbital images. A "consensus green", you might say. Or maybe I should say "hematite green". This may well be what Opportunity came to find.

I've long wrassled with another mystery that's both humbler and closer to home: Why the heck are NASA's color images of Mars so consistently bad? By "bad" I mean specifically "not as good as you get simply combining raw images without attempting to calibrate the color". To wit, here's an excerpt from yesterday's color panorama as envisaged by NASA:

(50%)

Compare it to the same image created by simply combining filters 4,5, and 6:

What the hell? I can't make the image look that bad even if I use the IR filter 2 for the red channel:

What the hell????? What is it with NASA and this fetish for a smoky dayglo brick-reddish-orange Mordor Mars?

Did I say "smoky"? Yeah...NASA's version is blurrier, and I have a hard time understanding how it could be due solely to geometric correction:

What...the...hell?!!

Tell me why, Dr. Nick! Make it stop hurting!